Power transformer is commonly used to convert the amplitude of the voltage of an alternating current (AC) source from one level to another. It plays an important role in electric power conversion, delivery, distribution, and utilization. In some implementations, the working principle of the power transformer is based on the magnetoelectric induction. When the primary windings of a power transformer are connected with an AC source, an AC voltage of the same frequency as that of the source voltage will be induced on the secondary windings. The voltage ratio of the power transformer, which is defined to be the ratio between the amplitudes of the input and output voltages, is determined by the turns ratio of the transformer. If the turns ratio of a power transformer is fixed, the voltage ratio will also be fixed. In some implementations, in order to change the voltage ratio of a power transformer, a tap-changing mechanism is used with the winding that allows a variable number of turns to be selected in discrete steps. The tap changer is a mechanical mechanism and can adjust the voltage ratio in discrete steps.
Power electronic converters can be used for variable-voltage and variable-frequency AC-AC power conversion. For example, power electronic converters can be made using power semiconductor devices, including insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), thyristors, and/or diodes, as well as passive components, such as inductors and capacitors. For example, AC-AC electric power conversion can be implemented using a variable frequency transformer (VFT). The VFT includes a rotary transformer (similar to an asynchronous generator) driven by an adjustable-speed DC motor drive. By adjusting the rotational speed of the VFT's rotor via the motor drive, two AC power systems with different frequencies or phases can be connected to the stator and rotor windings of the rotary transformer, respectively. The VFT can be used as a continuously variable phase-shifting transformer for power transfer between two asynchronous power networks with the same frequency.